Despite the importance of synaptic transmission in nervous system function, the mechanism of neurotransmitter release and the mechanisms of variable synaptic efficacy are poorly understood. The goals of the proposed research are to (1) test for differential effects of temperature, metabolic inhibitors, (Ca) and osmolarity on the magnitude and time course of facilitation and post-tetanic potentiation (PTP) of transmitter release during and following repetitive stimulation, (2) to determine if PTP results in whole or part from an increase in the store of transmitter available for release, (3) to determine if a feed-back system operates to regulate transmitter release, and (4) to determine if curare has a presynaptic effect on transmitter release. Depending on the experiment, estimates of the amount of transmitter released at the vertebrate neuromuscular junction will be made from measurements of end-plate potentials recorded intracellularly or extracellularly or extracellularly or from measurements of end-plate currents recorded under voltage clamp. Models for the mechanism of transmitter release in terms of the calcium hypothesis and quantal hypothesis will be formulated and evaluated by comparing experimental results to predicted results obtained by simulation of the models on a digital computer.